Further extending the meaning of `?` to check for nulls

Currently, `?` can be used to mean "if not null". For example:

```
// If user is not null, do something
user?.doSomething(123);
```

Which is equivalent to:

```
// If user is not null, do something
if (user != null) user.doSomething(123);
```

I propose to extend this a little bit. Instead of:

```
// If user is not null, and name is not null, do something
if (user != null && name != null) doSomething();
```

I'd like to write:

```
// If user is not null, and name is not null, do something
if (user? && name?) doSomething();
```

The issue names no files, tests, or entry points. Start by reviewing the existing null-aware `?` behavior and the proposed examples; done means deciding and documenting the syntax and semantics of `if (user? && name?)`.

Written by the indexing model from the issue text.